1. Field of the Invention
The present invention relates generally to a piezoelectric ceramic composition that is useful as a material for piezoelectric elements such as piezoelectric ceramic filters, piezoelectric ceramic vibrators, actuators, piezoelectric transformers, various types of sensors, or the like.
2. Related Background Art
Conventionally, as piezoelectric ceramic materials, lead titanate-based ceramics containing PbTiO, as a main component, lead zirconate titanate-based ceramics containing Pb(Ti, Zr)O3 as a main component, and a multicomponent piezoelectric ceramic that includes some kinds of complex perovskite compositions, for example, Pb(Mg1/3Nb2/3)O3, Pb(Ni1/3Nb2/3)O3 have been used. With these compositions, piezoelectric ceramics having different characteristics in accordance with the intended uses can be obtained through the suitable selection of a composition ratio of the components thereof. These piezoelectric ceramics are used for ceramic filters, piezobuzzers, piezoelectric spark plugs, ultrasonic piezoelectric transducers, or the like.
However, conventional lead zirconate titanate piezoelectric ceramics have a small frequency constant that is about 2000 Hzxc2x7m. Hence, when using such a piezoelectric ceramics, a resonator in which thickness longitudinal vibration occurs in the frequency band of about 10 MHz or higher has an element thickness of 0.2 mm or less, which makes the manufacture thereof difficult. In addition, the above-mentioned conventional materials contain a large amount of lead as a main component, which is not desirable from the viewpoint of environmental protection. The above-mentioned conventional materials have large grain sizes. Generally, materials with large grain sizes have a low mechanical quality factor Qm (hereinafter referred to as xe2x80x9cmechanical Qxe2x80x9d) and have a higher decreasing rate of frequency dependence in a high frequency range. Hence, it was difficult to use them for vibrators used in the high frequency range, for example, in a range of 1 MHz or higher.
Here, the xe2x80x9cmechanical Qxe2x80x9d is a factor indicating the sharpness of resonance in vibration, and the sharpness of a resonance curve increases with an increase in the value thereof.
Hence, an object of the present invention is to provide a piezoelectric ceramic composition that is manufactured easily by an ordinary sintering method and has a high coupling coefficient, a large frequency constant, a small grain size, and a high mechanical Q at least in its preferable embodiment. Another object of the present invention is to provide a piezoelectric element including this piezoelectric ceramic composition.
In one aspect of the present invention, a piezoelectric ceramic composition is expressed by a formula of (LixNa1xe2x88x92xxe2x88x92yKy)zxe2x88x922wMa2wNb1xe2x88x92wMbwO3.
In the above formula, 0.03xe2x89xa6xxe2x89xa60.2, 0xe2x89xa6yxe2x89xa60.2, 0.98xe2x89xa6zxe2x89xa61, 0 less than wxe2x89xa60.05, Ma indicates at least one element selected from the alkaline-earth metals, and Mb denotes at least one element selected from Bi, Sb, and the rare earth elements (Sc, Y. and lanthanide series elements).
In the above, the value of x is defined as 0.03xe2x89xa6xxe2x89xa60.2. This is because sintering characteristics deteriorate when the value of x is out of this range. The value of x is further preferably at least 0.07 and particularly preferably 0.17 or lower.
In the above, the value of y is defined as 0xe2x89xa6yxe2x89xa60.2. This is because the coupling coefficient may decrease when the value of y is out of this range. The increase in the value of y within this range permits an increase in dielectric constant.
In the above, the value of z is defined as 0.98xe2x89xa6zxe2x89xa61. This is because a value of z of lower than 0.98 does not allow the mechanical Q to be improved to a sufficiently high degree.
In the above, the value of w is defined as 0 less than wxe2x89xa60.05. This is because when the value of w is out of this range, the grain size may increase and the coupling coefficient may decrease. The value of w is preferably at least 0.001, further preferably at least 0.005, and particularly preferably 0.03 or lower.
The present invention further provides a piezoelectric element including the above-mentioned piezoelectric ceramic composition.